Buggy code:
public void updateDocuments ( Term delTerm , Iterable < ? extends Iterable < ? extends IndexableField > > docs , Analyzer analyzer ) throws IOException , CorruptIndexException { } public void deleteDocuments ( Term term ) throws IOException , CorruptIndexException { } public void deleteDocuments ( Term ... terms ) throws IOException , CorruptIndexException { } public void deleteDocuments ( Query query ) throws IOException , CorruptIndexException { } public void deleteDocuments ( Query ... queries ) throws IOException , CorruptIndexException { } public void updateDocument ( Term term , Iterable < ? extends IndexableField > doc ) throws IOException , CorruptIndexException { } public void updateDocument ( Term term , Iterable < ? extends IndexableField > doc , Analyzer analyzer ) throws IOException , CorruptIndexException { } final synchronized int getSegmentCount ( ) { } final synchronized int getNumBufferedDocuments ( ) { } final synchronized Collection < String > getIndexFileNames ( ) throws IOException { } final synchronized int getDocCount ( int i ) { } final int getFlushCount ( ) { } final int getFlushDeletesCount ( ) { } final String newSegmentName ( ) { } private PrintStream infoStream ; private static PrintStream defaultInfoStream ; public void optimize ( ) throws IOException , CorruptIndexException { } public void optimize ( int maxNumSegments ) throws IOException , CorruptIndexException { } public void optimize ( boolean doWait ) throws IOException , CorruptIndexException { } public void optimize ( int maxNumSegments , boolean doWait ) throws IOException , CorruptIndexException { } private synchronized boolean optimizeMergesPending ( ) { } public void expungeDeletes ( boolean doWait ) throws IOException , CorruptIndexException { } public void expungeDeletes ( ) throws IOException , CorruptIndexException { } public final void maybeMerge ( ) throws IOException , CorruptIndexException { } private final void maybeMerge ( boolean optimize ) throws IOException , CorruptIndexException { } private final void maybeMerge ( int maxNumSegmentsOptimize , boolean optimize ) throws IOException , CorruptIndexException { } private synchronized void updatePendingMerges ( int maxNumSegmentsOptimize , boolean optimize ) throws IOException , CorruptIndexException { } public synchronized Collection < SegmentInfo > getMergingSegments ( ) { } public synchronized OneMerge getNextMerge ( ) { } public void rollback ( ) throws IOException { } private void rollbackInternal ( ) throws IOException { } public synchronized void deleteAll ( ) throws IOException { } private synchronized void finishMerges ( boolean waitForMerges ) throws IOException { } public synchronized void waitForMerges ( ) { } synchronized void checkpoint ( ) throws IOException { } SegmentInfo prepareFlushedSegment ( FlushedSegment flushedSegment ) throws IOException { } synchronized void publishFlushedSegment ( SegmentInfo newSegment , FrozenBufferedDeletes packet , FrozenBufferedDeletes globalPacket ) throws IOException { } synchronized boolean useCompoundFile ( SegmentInfo segmentInfo ) throws IOException { } private synchronized void resetMergeExceptions ( ) { } private void noDupDirs ( Directory ... dirs ) { } public void addIndexes ( Directory ... dirs ) throws IOException , CorruptIndexException { } public void addIndexes ( IndexReader ... readers ) throws IOException , CorruptIndexException { } private void copySegmentIntoCFS ( SegmentInfo info , String segName , IOContext context ) throws IOException { String segFileName = IndexFileNames . segmentFileName ( segName , "" , COMPOUND_FILE_EXTENSION ) ; Collection < String > files = info . files ( ) ; final CompoundFileDirectory cfsdir = new CompoundFileDirectory ( directory , segFileName , context , true ) ; try { for ( String file : files ) { String newFileName = segName + ( IndexFileNames . stripSegmentName ( file ) ) ; if ( ( ! ( IndexFileNames . matchesExtension ( file , DELETES_EXTENSION ) ) ) && ( ! ( IndexFileNames . isSeparateNormsFile ( file ) ) ) ) { info . dir . copy ( cfsdir , file , file , context ) ; } else { assert ! ( directory . fileExists ( newFileName ) ) : ( "file<seq2seq4repair_space>\"" + newFileName ) + "\"<seq2seq4repair_space>already<seq2seq4repair_space>exists" ; info . dir . copy ( directory , file , newFileName , context ) ; } } } finally { <START_BUG> IOUtils . closeSafely ( true , cfsdir ) ; <END_BUG> } info . dir = directory ; info . name = segName ; info . setUseCompoundFile ( true ) ; } private void copySegmentAsIs ( SegmentInfo info , String segName , Map < String , String > dsNames , Set < String > dsFilesCopied , IOContext context ) throws IOException { } protected void doAfterFlush ( ) throws IOException { } protected void doBeforeFlush ( ) throws IOException { } public final void prepareCommit ( ) throws IOException , CorruptIndexException { } public final void prepareCommit ( Map < String , String > commitUserData ) throws IOException , CorruptIndexException { } private final Object commitLock = new Object ( ) ; public final void commit ( ) throws IOException , CorruptIndexException { } public final void commit ( Map < String , String > commitUserData ) throws IOException , CorruptIndexException { } private final void commitInternal ( Map < String , String > commitUserData ) throws IOException , CorruptIndexException { } private final synchronized void finishCommit ( ) throws IOException , CorruptIndexException { } private final Object fullFlushLock = new Object ( ) ; protected final void flush ( boolean triggerMerge , boolean applyAllDeletes ) throws IOException , CorruptIndexException { } private boolean doFlush ( boolean applyAllDeletes ) throws IOException , CorruptIndexException { } final synchronized void maybeApplyDeletes ( boolean applyAllDeletes ) throws IOException { } final synchronized void applyAllDeletes ( ) throws IOException { } public final long ramSizeInBytes ( ) { } DocumentsWriter getDocsWriter ( ) { } public final synchronized int numRamDocs ( ) { } private void ensureValidMerge ( MergePolicy . OneMerge merge ) throws IOException { } private synchronized void commitMergedDeletes ( MergePolicy . OneMerge merge , SegmentReader mergedReader ) throws IOException { } private synchronized boolean commitMerge ( MergePolicy . OneMerge merge , SegmentReader mergedReader ) throws IOException { }

Fix:
IOUtils . closeSafely ( false , cfsdir ) ;